Arginine (Arg, R)
Arginine is a basic amino acid whose guanidinium side chain stays positively charged across physiological pH. It drives electrostatic binding, membrane interaction and cell penetration in many peptides.
Structure and properties
Arginine has a long aliphatic side chain ending in a guanidinium group. The positive charge is delocalised across three nitrogens, so the group remains protonated under almost all conditions encountered in biology and in the laboratory. This planar, charge-spread structure lets arginine form multiple hydrogen bonds at once and pair strongly with carboxylates and phosphates, forming salt bridges that are often more stable than those made by lysine. Arginine is highly hydrophilic overall, but the aliphatic portion of the chain can pack against aromatic rings through cation-pi interactions. It is a key recognition residue for trypsin, which cleaves after Arg and Lys.
Role in peptides
Arginine-rich sequences are central to cell-penetrating peptides such as the HIV TAT fragment and oligoarginines, where the guanidinium groups interact with membrane phosphates and sulfated sugars. The Arg-Gly-Asp (RGD) motif mediates integrin binding. In melanocortin ligands, the His-Phe-Arg-Trp core is the recognised pharmacophore, preserved in Melanotan II and PT-141. SS-31 places a D-arginine at its N-terminus as part of an alternating cationic-aromatic motif, and Selank contains the tuftsin sequence Thr-Lys-Pro-Arg. Arginine also contributes much of the net positive charge of LL-37.
Stability and analysis
Arginine is chemically robust in solution. Most issues arise in synthesis and formulation. The side-chain protecting group (commonly Pbf) can be removed incompletely during cleavage, giving adducts visible by mass spectrometry, and its by-products can alkylate nearby tryptophan unless scavengers are used. Activated arginine can cyclise to a delta-lactam during coupling, leading to deletion sequences. Each basic residue carries a counter-ion, so arginine-rich peptides hold a substantial fraction of trifluoroacetate or acetate by weight, which lowers net peptide content.
In PeptideWiki compounds
Compounds whose sequence contains arginine, with the number of occurrences.
- IGF-1 LR3 (7)
- Tesamorelin (6)
- LL-37 (5)
- Abaloparatide (3)
- CJC-1295 (3)
- Humanin (3)
- MOTS-c (3)
- Sermorelin (3)
- AOD-9604 (2)
- Argireline (acetyl hexapeptide-8) (2)
- Setmelanotide (2)
- Snap-8 (2)
- Teriparatide (2)
- VIP (vasoactive intestinal peptide) (2)
- ARA-290 (cibinetide) (1)
- Afamelanotide (1)
- Calcitonin (salmon) (1)
- Exenatide (1)
- Gonadorelin (1)
- Kisspeptin-10 (1)
- Melanotan II (1)
- PT-141 (bremelanotide) (1)
- Pinealon (1)
- Pramlintide (1)
- SS-31 (elamipretide) (1)
- Selank (1)
- Triptorelin (1)
Frequently asked questions
Why do arginine-rich peptides enter cells?
The guanidinium group forms bidentate hydrogen bonds with phosphate head groups and sulfated glycosaminoglycans on the cell surface. This binding promotes uptake through endocytosis and, under some conditions, direct membrane translocation. Arginine is more effective than lysine in this role, which is why oligoarginine and TAT-derived sequences are widely used in vitro as carriers for cargo molecules. Efficiency depends strongly on cell type, concentration and the attached cargo.
Does arginine affect counter-ion content?
Yes. Every arginine, lysine and histidine, plus a free N-terminus, can pair with a counter-ion from purification, usually trifluoroacetate or acetate. Peptides with many arginines therefore contain a larger mass fraction of salt, so the net peptide content is lower than the gross weight suggests. A certificate of analysis should state the counter-ion and ideally the net peptide content. See the counter-ion guide for detail.
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